An optical system utilizing polarized light, i.e., polarizing optical system, has recently been used in various fields including a liquid crystal display projector. For example, spatial light modulators which modulates polarized light spatially and polarizing beam splitters which separate S polarized light from P polarized light are used for a liquid crystal display projector. In such polarizing optical system, there is growing demand for more accurate control of polarization characteristics.
Among optical parts for the polarizing optical system, parts such as prisms and substrates of light polarizing elements used for a polarizing beam splitter or a spatial light modulator which requires stable maintenance of polarization characteristics must have optical isotropy, since if a material having optical anisotropy is used for these parts, the phase difference (optical path difference) between the ordinary ray and the extraordinary ray with respect to light which has been transmitted by the optical element, and therefore the polarizing characteristic cannot be retained in such a case.
Even if a prior art optical glass which is sufficiently annealed and removed of strain and has optical isotropy is used as an optical part of the polarizing optical system for which maintenance of polarization characteristics is required, such part will exhibit optical anisotropy called birefringence which is caused by a photoelastic effect when mechanical stress or thermal stress is applied to such part, if such part has a large photoelastic constant and, as a result, desired polarization characteristics cannot be obtained. The mechanical stress is produced by, for example, joining a material which has a different thermal expansion coefficient from that of the optical glass with an optical glass. The thermal stress is produced by, for example, heat generated by a peripheral equipment or by heat generated by the optical glass due to absorption of energy of transmitting light. Magnitude of birefringence which the optical glass exhibits upon application of such stress can be expressed by difference in optical path difference. The following formula (1) is established when optical path difference is expressed by σ(nm), thickness of the optical glass by d(cm) and stress by F(Pa), indicating that birefringence increases with optical path difference.σ=β·d·F  (1)
The proportion constant β in the above formula (1) is called photoelastic constant the value of which differs by the type of the glass. As shown by the above formula (1), when the same stress (F) applied to two glasses which have the same thickness (d), a glass having a smaller absolute value of photoelastic constant (β) has a smaller optical path difference (σ), i.e., smaller birefringence.
Contribution of photoelastic constant to parts for polarizing optical system is described in, for example, Japanese Patent Application Laid-open Publication No. Hei 7-306314, No. Hei 8-234179 and No. Hei 9-127461. As for more specific numerical analysis, Japanese Patent Application Laid-open Publication No. 2000-171770 discloses that parts satisfying the following formula are desirable in the polarizing optical system. The right side of this formula indicates amount of birefringence caused by thermal stress.5.00×102≧K·α·E·∫(1−T)dλ/ρ/Cp  (2)
where K represents photoelastic constant (nm/mm·mm2/N), αcoefficient of linear thermal expansion (10−6/K), E Young's modulus(103N/mm2), λ wavelength of light used, T internal transmittance of the part at the wavelength λ, ρ gravity of the part (g/cm3), and Cp specific heat (J/g·k)
In the formula (2), the range of integration is main absorption wavelengths, i.e., 420 (nm) to 500 (nm). It is apparent from the formula (2) that birefringence caused by thermal stress can be reduced as the absolute value of photoelastic constant approaches zero.
Photoelastic constant has wavelength dependency and therefore it is not constant over all visible region (400 nm–700 nm). Therefore, if the wavelength dependency is large, the polarization characteristic is not uniform over all visible region, even if photoelastic constant at wavelength 546 nm which is a representative value in the visible region is small. For example, an optical glass having a high PbO concentration as disclosed in Japaanese Patent Application Laid-open Publication No. Hei 11-133528 has wavelength dependency according to which photoelastic constant decreases toward a shorter wavelength and amount of change  β in photoelastic constant in the wavelength range of 400 nm–700 nm becomes about 0.8×10−5 nm/cm/Pa. When such optical glass is used, for example, as a polarizing beam splitter for a liquid crystal display projector, photoelastic constant β is 0.0×10−5 nm/cm/Pa at green (G) light (wavelength in the vicinity of 550 nm) but the absolute value of β becomes about 0.4×10−5 nm/cm/Pa at blue (B) light (wavelength in the vicinity of 430 nm) and red (R) light (wavelength in the vicinity of 640 nm). Particularaly, since it has a large refractive index in B light, optical path difference due to birefringency becomes large in B light.
Aside from the above described problem of birefringence, an optical glass used for application in which temperature of optical parts changes largely depending upon the environment in which it is used (e.g., heat generation by a prism and lens and peripheral electrical circuits and parts exposed to irradiated light of high intensity) must have a high internal transmittance for preventing elevation of temperature in optical parts due to absorption of irradiated light at high intensity.
Known as optical glasses having a small photoelastic constant which are useful as parts for polarizing optical system are B2O3—Al2O3—PbO glasses as disclosed in Japanese Patent Application Laid-open Publication No. Hei 9-48631, phosphate glasses comprising BaO and/or PbO as disclosed in Japanese Patent Applicatin Laid-open Publication No. Hei 11-335135, phosphate glasses comprising no PbO as disclosed in Japanese Patent Application Laid-open Publication No. Hei 11-199269, No. 2000-34132, No. 2002-128540 and No. 2002-338294, and fluoro-phosphate glasses as disclosed in Japanese Patent Application Laid-open Publication No. Hei 9-48633.
As disclosed in literature such as Journal of The Society of Glass Technology (1957) 353T-362T “The Effect of the Polarisation of the Constituent Ions on the Photoelastic Birefringence of the Glass” by Megumi Tashiro, it has been well known that PbO among glass constituents has a large effect of decreasing photoelastic constant. There are various glasses utilizing this effect to cause the glasses to have a small photoelastic constant but, for obtaining a glass having an extremely small photoelastic constant, these glasses must comprise a large amount of a Pb compound such, for example, as PbO and PbF2 which imposes a heavy burden to the environment. This is likely to cause adverse effects to the environment and, therefore, it is not desirable to put such glasses to practical application.
It is disclosed in Journal of the American Ceramic Society Vo.68 (1985) P389-P39 “Photoelastic effects in Phosphate Glasses” by Matsushita et al that in phosphate glasses, particularly P2O5—BaO glasses have a relatively small photoelastic constant. However, for reducing photoelastic constant to below 0.3×10−5 nm·cm−1·Pa−1 by the method of utilizing the photoelastic constant reduction effect by the BaO component only, a special manufacturing process such as super quenching method must be employed because of limitation in the vitrification range of the P2O5—BaO glasses. This method is of a poor productivity and, moreover, it is difficult to form the glass to a desired optical parts by this method.
Japanese Patent Applicatin Laid-open Publication No. 2002-338294 discloses a method for manufacturing P2O5—BaO glasses comprising a large amount of BaO and examples having a very small photoelastic constant of 0.15×10−12(1/Pa) are described in this publication. For obtaining these glasses, however, a method of blowing plural types of gases such as humid air and chlorine gas into melted glass must be employed and this method is of a poor productivity.
Japanese Patent Application Laid-open Publication No. Hei 11-199269 and No. 2000-34132 disclose that, in phosphate glasses which are free of Pb compounds, not only BaO but also La2O3 is effective for reducing photoelastic constant. It is apparent from examples of these publications that it is difficult to produce glasses having a very small photoelastic constant, particularly glasses having photoelastic constant of 0.3×10−5 nm·cm−1·Pa−1 or below in the visible region, by the effect of BaO and La2O3.
Japanese Patent Application No. 2003-110394 discloses that it is possible to produce phosphate glasses having photoelastic constant of 0.3×10−5 nm·cm−1·Pa−1 or below. There is, however, the problem that when phosphate glasses containing a large amount of PbO and BaO is used, platinum or platinum group ions such as palladium and rhodium come out of a platinum crucible during the indispensable manufacturing process of melting of the glass in the platinum crucible with the result that the glass is colored by platinum or platinum group ions. According to Journal of the American Ceramic Society Vo. 39 (1956) P173–P180 “The Colors of Platinum, Palladium, and Rhodium in simple Glasses” by G. E. Rindone et. al., it has become apparent that absorption of platinum ions in phosphate glasses occurs significantly in the vicinity of 400–500 nm. Presence of such absorbing ions causes light absorbed when light of wavelength of 400–500 nm transmits through an optical part to be converted to heat with resulting generation of heat which increases thermal stress in the optical part. When such glass is used for an optical part such as lens or prism in which light transmits through the entire visible region, blue light of 400–500 nm is selectively absorbed with the result that light which has transmitted through the optical part exhibits a yellowish color. Since a glass which has a smaller degree of unnecessary light absorption can realize more excellent properties, though the level of requirement for such condition differs depending upon the purpose of the glass, a glass with a high internal transmittance is desirable. In other words, in manufacturing phosphate glasses containing a large amount of PbO and BaO, much care must be taken for prevention of coming out of platinum group ions.
As disclosed by commercially available fluoro-phosphate glasses (e.g., S-FPL51, S-FPL52 and S-FPL53 manufactured by Ohara Corporation), Japanese Patent Application Laid-open Publication No. Hei-9-48633, U.S. Pat. No. 5,969,861, DE19631171A1 and Japanese Patent Application Laid-open Publication No. Hei 11-60267, bond in fluoro-phophate glasses is generally not covalent bond as in Si—O but mostly ionic bond and hence change in the electron structure of the glass due to stress is relatively small with the result that these glasses have a relatively small photoelastic constant. Moreover, in fluoro-phosphate glasses, melting of the glass at a relatively low temperature is possible and, therefore, there occurs little or no coming out of platinum group ions from a melting equipment made of platinum such as a crucible and a glass stirring device which takes place in phosphate glasses containing a large amount of PbO and BaO. As a result, there is no problem of selective absorption by the platinum group ions in the visible region (particularly at 400–500 nm).
Besides these fluro-phosphate glasses, known in the art are fluoro-phosphate glasses having a refractive index (nd) in the vicinity of 1.60–1.68 and Abben number (ν d) in the vicinity of 40–65, though no mention is made about photoelastic constant, as disclosed in Japanese Patent Application Laid-open No. Sho 50-50416, No. Sho 57-123842, Sho 59-18133 and No. Hei 2-124740 which are characterized by having a large anomalous dispersion characteristic and fluoro-phosphate glasses as disclosed in Japanese Patent Application Laid-open Publication No. Hei 6-157068 and No. Hei 10-53434 which have excellent resistance to devitrification and good melting property.
Fluoro-phosphate glasses generally have a refractive index (nd) of less than 1.6 or Abbe number (ν d) of 65 or over, as shown in Japanese Patent Application Laid-open Publication No. Hei 2-124740, No. Hei 6-157068 and Hei 9-48633. For increasing refractive index (nd), in fluoro-phosphate glasses disclosed in Japanese Patent Application Laid-open Publication No. Sho 50-50416, No. Sho 57-123842, No. Sho 59-18133 and No. Hei 11-60267, total amount of fluoride materials is restricted to less than 45% and resulting examples having a refractive index exceeding 1.6 are described. In this case, however, restriction to introduction of fluorides in the glass increases covalent bond in the glass with resulting difficulty in realizing a glass having a desired small photoelastic constant. Further, since the glass of Japanese Patent Application Laid-open Publication No. Hei 10-53434 contains Al2O3 in high amount of 21.23–26.35 weight % and also contains MgF2 as an essential ingredient, this glass is expected to have a large photoelastic constant.
As fluoro-phosphate glasses for different purposes, fluoro-phosphate glasses for filters containing CeO2 or CuO which is an ingredient which absorbs light of specific wavelength as an essential ingredient are disclosed in Japanese Patent Application Laid-open Publication No. Hei 1-219038, No. Hei 3-83835, No. Hei 3-83834 and No. Hei 4-214043. As for Japanese Patent Application Laid-open Publication No. Hei 1-219038, No. Hei 3-83835 and No. Hei 3-83834, most examples of these glasses contain P2O5 in a high amount of 20% or more and hence there is high possibility that a desired photoelastic constant cannot be realized by these glasses. In examples of these glasses which contain P2O5 in an amount of less than 20%, they contain MgF2 which relatively increases photoelastic constant in a high amount (Example No. 1 of Japanese Patent Application Laid-open Publication No. Hei 1-219038) or contain B2O3 which increases photoelastic constant in a high amount (Example Nos. 2, 4 and 10 of Japanese Patent Application Laid-open Publication No. Hei 3-83835 and Example No. 2 of Japanese Patent Application Laid-open Publication No. Hei 3-83834). Examples of Japanese Patent Application Laid-open Publication No. Hei 4-214043 contain a large amount of MgF2 and hence cannot realize a desired photoelastic constant and, moreover, these glasses are likely to have a low refractive index.
It is, therefore, an object of the present invention to provide an optical glass suitable for parts for polarizing optical system and light polarization control elements which has a small photoelastic constant and also has properties such as refractive index (nd) and Abbe number (ν d) which are useful for purposes of optical glasses.
It is another object of the present invention to provide an optical glass as described above which is free of Pb compounds.